That sweet line, drawn across the top of the wall, sweepingly level, taking your eyes for a walk along the zig-zaggingly, unsagging line of stones that run parallel to the level of everything we know to be level, and ever was.
Those tidy tops of the otherwise untidy last-stones-to-be-found, all snuggled together now along one tidy course, all marching in formation, it's all so improbably, perfect
Those remaining shapes, fortuitously left in the pile after most of the wall is built, now lifted into place, turned upright, bedded and wedged, becoming the crowning toping of the dry stone layer-cake we build, or rather, are baking .
This is the serendipitous cope-aesthetic phenomena.